linux-kernel - Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

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Message-ID: <25b7aa40-fad0-4886-90b2-c5d68d75d28b@lucifer.local>
Date:   Tue, 2 May 2023 16:19:10 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Matthew Wilcox <willy@...radead.org>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Leon Romanovsky <leon@...nel.org>,
        Christian Benvenuti <benve@...co.com>,
        Nelson Escobar <neescoba@...co.com>,
        Bernard Metzler <bmt@...ich.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
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        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
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        Magnus Karlsson <magnus.karlsson@...el.com>,
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        John Fastabend <john.fastabend@...il.com>,
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        Oleg Nesterov <oleg@...hat.com>,
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        Dave Chinner <david@...morbit.com>,
        Theodore Ts'o <tytso@....edu>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings

On Tue, May 02, 2023 at 05:09:06PM +0200, David Hildenbrand wrote:
> On 02.05.23 15:56, Matthew Rosato wrote:
> > On 5/2/23 9:50 AM, Jason Gunthorpe wrote:
> > > On Tue, May 02, 2023 at 03:47:43PM +0200, David Hildenbrand wrote:
> > > > > Eventually we want to implement a mechanism where we can dynamically pin in response to RPCIT.
> > > >
> > > > Okay, so IIRC we'll fail starting the domain early, that's good. And if we
> > > > pin all guest memory (instead of small pieces dynamically), there is little
> > > > existing use for file-backed RAM in such zPCI configurations (because memory
> > > > cannot be reclaimed either way if it's all pinned), so likely there are no
> > > > real existing users.
> > >
> > > Right, this is VFIO, the physical HW can't tolerate not having pinned
> > > memory, so something somewhere is always pinning it.
> >
> > I might have mis-explained above.
> >
> > With iommufd nesting, we will pin everything upfront as a starting point.
> >
> > The current usage of vfio type1 iommu for s390 does not pin the entirety of guest memory upfront, it happens as guest RPCITs occur / type1 mappings are made.
>
> ... so, after the domain started successfully on the libvirt/QEMU side ? :/
>
> It would be great to confirm that. There might be a BUG in patch #2 (see my
> reply to patch #2) that might not allow you to reproduce it right now.
>

Yes apologies - thank you VERY much for doing this Matthew, but apologies, I
made rather a clanger in patch 2 which would mean fast patch degrading to slow
path would pass even for file-backed.

Will respin a v7 + cc you on that, if you could be so kind as to test that?

> --
> Thanks,
>
> David / dhildenb
>